My fave: The 2001 CO bonus machine: CO OnePass flyers could sign up for multiple AMEX Double Miles promos and assorted bonuses that resulted in bonuses as high 5 to 7 times the mileage flown (some bragged of even more!). And it lasted for several months. Great times those were.

Also got in on the $20 British Airways USA-Europe WT+ (premium econ)